Selective etch process

ABSTRACT

Disclosed is an etch process wherein hydrogen monoiodide (HI) ions are employed to bombard a patterned film, thereby creating geometric features in the patterned film with substantially anisotropic sidewalls. The etch process has a high selectivity to oxide, allowing the etch process to terminate on a thin pad oxide, especially when using a two step etch process. The etch process is also highly selective to photoresist, further enhancing the resulting anisotropic nature of the geometrical feature sidewalls.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates to semiconductor device manufacturing.More particularly, the present invention is directed to a process forenhancing etch selectivity and uniformity when etchingsilicon-containing materials in semiconductor device manufacturing.

2. The Relevant Technology

The integrated circuit manufacturing industry is rapidly progressingtoward more highly advanced and miniaturized integrated circuits. Thisprogress is effectively revolutionizing the electronics industry due tothe higher capability of the electronic devices that can be produced asa result. In order to continue this progress, however, new manufacturingprocesses are needed which have the capability of producing the finerfeatures that are required. For instance, improved processes are neededto etch substantially normal or anisotropic submicron sidewalls onsilicon substrates of in-process integrated semiconductor circuits. Newprocesses are also needed to etch submicron geometric features in filmson the silicon substrates to exact depths. Consequently, the newprocesses must have a high selectivity to photoresist and to pad oxides.

The prior art has utilized methods such as the use of diatomic chloridein ionized plasma etches to emphasize the physical aspects of the etch,so as to produce more anisotropic sidewalls. Etch processes have evenbeen conducted in two stages, a primary etch and a secondary etch, withthe secondary etch using less active compounds. However, as featuresbecome smaller, even these processes are proving insufficient.

One example of a drawback of current prior art processes is shown inFIG. 1. Therein is shown a line space pair 12 being etched in a film ofpolysilicon 14. Polysilicon film 14 is formed over a pad oxide 16 on asilicon substrate 10. Above polysilicon film 14 is a patterned layer ofphotoresist 18. It can be seen from FIG. 1 that the ionized etchantmaterial attacks the exposed surface of polysilicon film 14.

The prior art etch also has a tendency to attack the sidewalls of linespace pair 12. This results in undercutting of polysilicon film 14beneath photoresist layer 18. Furthermore, when using highly reactiveetchants such as diatomic chloride, the sidewalls of photoresist layer18 are also eroded, thereby giving the etchant even greater access tothe sidewalls 20 of line space pair 12, and resulting innon-anisotropically sloped sidewalls. This is problematic, as it altersthe critical dimensions of the device features. Also, thenon-anisotropically sloped sidewalls are undesirable, as it oftennecessary when forming certain features of integrated circuits, such asgate regions in DRAM memory cells, to etch lines and other graphicfeatures anisotropically. Anisotropic etches are also desirable as theyallow for higher device density and consequently, greater integratedcircuit miniaturization.

Thus, it can be seen that a need exists in the art for an etch processwith which selectivity to both photoresist and oxide can be increased,and which results in a more anisotropic nature of sidewalls in etchedgeometrical features.

SUMMARY AND OBJECTS OF THE INVENTION

The present invention seeks to resolve the above and other problemswhich have been experienced in the art. More particularly, the presentinvention constitutes an advancement in the art by providing a highlyselective etch process which achieves each of the objects listed below.

It is an object of the present invention to provide an etch processwhereby silicon-containing films such as polysilicon, single crystalsilicon, or silicon nitride can be etched in a substantially anisotropicmanner.

It is also an object of the present invention to provide such an etchprocess which has high selectivity to photoresist for maintaining ananisotropic etch process.

It is another object of the present invention to provide such an etchprocess which has high selectivity to oxides.

It is likewise an object of the present invention to provide an etchprocess which forms a strong passivation layer on sidewalls of thegeometrical features being etched, with the result of substantiallyanisotropic sidewalls.

It is further an object of the present invention to provide such an etchprocess whereby precision etching can be conducted using a two stepprocess.

To achieve the foregoing objects, and in accordance with the inventionas embodied and broadly described herein in the preferred embodiment, anetch process is provided which utilizes ions of an iodide containingcompound such as hydrogen monoiodide (HI) ions to bombard a patternedfilm on a silicon substrate of an in-process integrated semiconductorcircuit. The etch process thereby creates geometrical features in thefilm that have substantially anisotropic sidewalls. The etch process hasa high selectivity to oxide, making it easy to stop on a thin pad oxide.The etch process also has a high selectivity to photoresist, furtherenhancing the anisotropic nature of the resulting etched geometricalfeature sidewalls. Due to the high selectivity to photoresist, thinnerphotoresist mask layers can be used, thereby enhancing thephotolithography process.

The first step of the process of the present invention comprisespreparing the film that is to be etched. This typically comprisesforming an in-process integrated semiconductor circuit and depositingthe film thereon. Next, a photoresist layer is formed on the film andpatterned. In a further step, the in-process integrated semiconductorcircuit is placed in a reactive ion etching chamber or other suchchamber, the chamber is evacuated, and the film is exposed to ionizedetchants.

Preferably, the film is exposed to ionized etchants in two operations, aprimary etch and a secondary etch. The primary etch is conductedutilizing a highly reactive etchant such as chlorine, and is conductedfor the majority of the etch distance. Typically, in etching line andspace openings at current depths, the primary etch is conducted for atime sufficient to etch between about 5,000 and 7,000 Angstroms. About300 to 400 Angstroms are left to be etched by the secondary etch. Thesecondary etch typically comprises a halogen-containing compound such asNF₃, Cl₂, or HBr in combination with hydrogen monoiodide (HI).

The HI used in the secondary etch is a very passive etchant, and alsoforms a byproduct in the form of a passivation layer comprising asilicon iodide (SiI) containing polymer, which forms on sidewalls of thegeometrical features being etched. A high energy is required topenetrate the passivation layer on the sidewalls. The energy of the ionbombardment is sufficient to etch past the passivation layer at therespective bottoms of the sidewalls, but the ion bombardment does nothave access to the sidewalls due to a substantially normal angle ofincidence. Further, the ion bombardment is unable to penetrate thepassivation layer on the sidewalls. Consequently, a substantiallyanisotropic sidewall results.

Furthermore, due to the high selectivity to oxide of the etch process,it is easy to terminate the etch at an exact depth by providing a padoxide at the bottom of the line space pair. Furthermore, as thephotoresist is not substantially etched, the critical dimensions of thegeometrical features are maintained, and the sidewalls of thegeometrical features are further protected from being etched.

The etch is preferred for use with silicon-containing compounds such aspolysilicon, single crystal silicon, silicon nitride, and refractorymetal silicides.

Thus, an etch process is provided which can be used to etch highlyanisotropic sidewalls of geometrical features in silicon-containingcompounds. The etch process has high selectivity to oxide in order tostop at exact depths, and also has high selectivity to photoresist suchthat the critical dimensions and anisotropic nature of the sidewalls arefurther maintained.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the manner in which the above-recited and other advantagesand objects of the invention are obtained will be understood, a moreparticular description of the invention briefly described above will berendered by reference to a specific embodiment thereof which isillustrated in the appended drawings. Understanding that these drawingsdepict only a typical embodiment of the invention and are not thereforeto be considered to be limiting of its scope, the invention will bedescribed and explained with additional specificity and detail throughthe use of the accompanying drawings in which:

FIG. 1 is a schematic cross-sectional representation of an in-processintegrated semiconductor circuit of the prior art undergoing an etchoperation and showing the prior art problem of undercutting.

FIG. 2 is a schematic cross-sectional representation of an in-processintegrated semiconductor circuit shown prepared for the etch process ofthe present invention with a patterned photoresist layer.

FIG. 3 is a schematic cross-sectional representation of the in-processintegrated semiconductor circuit of FIG. 2, shown subsequent to an etchprocess conducted under the present invention.

FIG. 4 is a schematic cross-sectional representation of the in-processintegrated semiconductor circuit of FIG. 3, shown after the inventiveetch process has been conducted and the photoresist layer has beenstripped.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention comprises the use of an etchant which emphasizesthe physical aspect of an ion plasma etch process. The result is afocusing of the etch on exposed surfaces and the formation of aneffective passivation layer on the sidewalls of the geometrical featuresbeing formed. This serves to form more anisotropic geometrical features.Also, as the etch process has high selectivity to oxides, the etchprocess results in more precise formation of the geometrical featuresand greater control of the depth of the etch.

FIG. 2 illustrates the first step of the present invention whichcomprises preparing a surface to be etched. This typically comprisesforming a surface such as a polysilicon or other silicon-containinglayer on a silicon substrate 10 of an in-process integratedsemiconductor circuit. Such a structure is shown in FIG. 2, where thesurface to be etched comprises a polysilicon layer 14. Polysilicon layer14 is masked with a photoresist layer 18 using photolithography. A padoxide 16 can be formed on silicon substrate 10 before depositingpolysilicon layer 14 in order to set the depth of the etch process.

A further step comprises placing the in-process integrated semiconductorcircuit in an etch chamber. One possible etch chamber comprises anApplied 5000 magnetically enhanced reactive ion etcher sold by AppliedMaterials Corporation of Santa Clara, Calif., USA. The etch chamber isthen evacuated to a low pressure. Next, an etchant is flowed into thechamber and the in-process integrated semiconductor circuit is exposedto the etchant. A bias is then applied to the in-process integratedsemiconductor circuit or to the holder of the wafer on which is situatedthe in-process integrated semiconductor circuit This step is illustratedin FIG. 3.

The in-process integrated semiconductor circuit is preferably exposed tothe etchant in a primary etch and a secondary etch. The primary etchpreferably has a high etch rate, while the secondary etch has a loweretch rate and a high selectivity to oxide. Examples of the etchant usedin the primary etch include diatomic chloride, and fluorine-containingcompounds such as NF₃. The primary etch may also contain a portion of aniodide containing compound such as hydrogen monoiodide (HI). Thesecondary etch preferably uses an etchant comprising an iodidecontaining compound such as hydrogen monoiodide and a compoundcontaining diatomic chloride, fluorine, or bromine.

The primary etch is preferably conducted by flowing diatomic chlorine(Cl₂) at a rate of approximately 45 SCCM, and a fluorine-containingcompound, preferably nitrogen fluoride (NF₃), at a rate of approximately5 SCCM for a time in the approximate range of 30 to 60 seconds. Thepreferred pressure when using the Applied 5000 etcher is in theapproximate rage of 100 milliTorr and the preferred power isapproximately 500 watts. One of ordinary skill in the art willunderstand that the above values will vary depending on the make andmodel of the etcher used in the process. An inert gas, preferably argon(Ar) can also be added to the etch plasma. The inert gas tends tofurther enhance the uniformity of the etch process. Argon is preferredbecause of its weight and commercial availability, but other inertgasses can also be used.

It should also be noted that because of the high Cl₂ ion bombardment,the Cl₂ flow must be stopped before the exposed features are clearedfrom the silicon substrate. If the features are allowed to be cleared,the high ionic bombardment will cause the Cl₂ to penetrate the oxidelayer. Hence, the primary etch is stopped while there is a fair amountof film left on the in-process integrated semiconductor circuit. Thus,one drawback of such a physical Cl₂ etch is that it has a very poorselectivity to the underlying pad oxide. To substantially alleviate thisproblem, the process of the present invention employs a highly selectivesecondary etch. The secondary etch, also known as the "overetch,"comprises flowing a fluorine-containing compound or chlorine, flowed ata rate of approximately 30 SCCM along with the HI flowed at a rate ofapproximately 15 SCCM. The secondary etch is timed and typically lastsabout 35 seconds. The preferred pressure is about 300 milliTorr, and thepreferred power is about 100 watts. Again, process parameters will varywith the make and model of the etcher employed in the process.

During the etch process, the primary etch etches down about 5,000-7,000Angstroms into polysilicon layer 14, and typically leaves about 300 to400 Angstroms to be etched. Thus, once the etch gets within about 300 to400 Angstroms of the pad oxide, the primary etch is stopped and thesecondary etch is applied. At this point, the etch becomes largelyattributable to the physical component of the etch process, due to theselection of HI as an etchant. The high passivity of the HI helps togive a high selectivity to the underlying oxide. The HI is also a higherorder halogen, and consequently combines to form a strong passivationlayer 24 on sidewalls 22 being etched, to result in geometrical features15, seen in FIG. 4.

Passivation layer 24 comprises some form of silicon iodide (SiI)containing polymer. In order to etch through passivation layer 24, highenergy from the impinging ions of the ion bombardment are necessary.Without this added energy, the etch cannot pass through passivationlayer 24. The nature of ion bombardment is that the ions bombard at asubstantially normal angle of incidence. Thus, only the exposed surfacesof geometrical features such as the bottom of the line space pair 12illustrated in FIG. 4, are exposed to the energy of the ions. Sidewalls22, which form the periphery of line space pair 12, are not exposed, andthus passivation layer 24 prevents etching into sidewalls 22 by theactive chemical compounds. Consequently, sidewalls 22 are notsubstantially etched, and the etch produces substantially anisotropicsidewalls 22.

Furthermore, sidewalls 22 of photoresist layer 18 are also passivated,and as the etchant is highly selective to photoresist layer 18,photoresist layer 18 is not substantially etched away, thus furtherhelping to maintain the critical dimensions and anisotropic nature ofgeometrical features such as line space pair 12 on polysilicon layer 14.

The amount of ion bombardment and the power applied in creating the biasis selected in order to attain the optimum amount of selectivity. Theoxide has a greater bond strength than films such as polysilicon andnitride. Thus, when the proper amount of power is applied, the bonds arebroken on the polysilicon and the nitride but are not substantiallybroken on the pad oxide. This is especially advantageous with the use ofpolysilicon, for which it is difficult to correctly set the energy levelsuch that undercutting into sidewalls 22 does not occur. Because of thegreater passivity and slower etch rate, as well as high selectivity tooxide of the iodine in the HI, it is easier to set this level of power.

The compound NF₃ is advantageous for use with the secondary etch, as itgives a very high silicon and nitride etch rate but is still selectiveto oxide and photoresist when combined with HI. The compound SF₆ hasalso been found to be advantageous for use in the secondary etch. UsingSF₆ and HI, the etch rate of the secondary etch will be in a range ofabout 1,000 to about 1,500 Angstroms per minute. Thus, with this slowetch rate it is easy to completely etch the line space pair such as linespace pair 12 but not etch substantially into the underlying pad oxidelayer, particularly when the secondary etch is limited to about 300 to400 Angstroms.

The results of the secondary etch step are shown in FIG. 3, where it canbe seen that anisotropic sidewalls 22 have resulted, and essentially noundercutting occurs in fully etched line space pair 12 seen in FIG. 4.Thus, submicron features can be etched with considerable reliability.Furthermore, the photoresist pattern exhibits less faceting than withmethods of the prior art, consequently having less impact on theresulting film features.

The final step is to remove the photoresist from the film surface. Theresulting structure is shown as line space pair 12 in FIG. 4.

The etch process is not limited to the two step embodiment discussedabove. Alternatively, the etch could be conducted in a single step. Forinstance, the etch could be conducted in a single step with afluorine-containing compound and HI. Also, the etch could conceivablycomprise solely HI in a single step, or HI could be used alone in one ofmultiple steps.

Other etch chambers can also be used with the etch process of thepresent invention. For instance, a reactive ion etcher (RIE) such as theLam Rainbow available from Lam Research of the city of Fremont, in thestate of California, USA, could be used and would be preferably set witha high pressure of between 200 milliTorr and 500 milliTorrAlternatively, a high density etcher such as the Lam 9400 high densityetcher, also available from Lain Research, can also be used, and ispreferably set with a pressure of between about 20 and 80 milliTorr. Thehigh density etcher should also be set with a pressure of about 10milliTorr, a power in a range of about 300-900 watts at the topelectrode, and a power in a range of about 150-225 watts at the bottomelectrode.

Films which are preferred to be etched by the process of the presentinvention comprise polysilicon, as well as single crystal silicon,silicon nitride and refractory metal silicides such as tungstensilicide, molybdenum silicide, and cobol silicide. Essentially the onlysilicon-containing material which it is predicted would not be etchedeffectively is silicon dioxide, due to its high bond strength.

Specific applications for the etch include etching polysilicon over verythin gate oxides. One advantage is that vertical profiles aremaintained. The etch process can also be used to enhance the selectivitybetween polysilicon and photoresist. Also, the etch process can be usedto etch nitride features having underlying pad oxides with a highselectivity to the underlying pad oxides.

Thus, a process is provided with which silicon-containing films can beetched with high selectivity to oxide and photoresist, and whereby apassivation layer will be formed which requires a sufficiently highionization energy to penetrate feature sidewalls so that the sidewallswill not be substantially etched but rather will be formed in a highlyanisotropic manner.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrated andnot restrictive. The scope of the invention is, therefore, indicated bythe appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed and desired to be secured by United States LettersPatent is:
 1. An etch process which forms substantially anisotropicsidewalls and which has a high selectivity to oxide and photoresist, theetch process comprising:(a) preparing a pattered film to be etched on anin-process integrated semiconductor circuit; and (b) exposing the filmto ionized hydrogen monoiodide (HI), the film being selected from thegroup consisting of single crystal silicon, polysilicon, siliconnitride, and refractory metal silicide, whereby a geometric feature isformed in the film, the geometric feature defining an anisotropicsidewall.
 2. An etch process as recited in claim 1, wherein thepatterned film is exposed to an iodide containing compound incombination with a fluorine-containing compound.
 3. An etch process asrecited in claim 1, wherein the patterned film is exposed to an iodidecontaining compound in combination with a chlorine-containing compound.4. An etch process as recited in claim 1, wherein the patterned film isexposed to an iodide containing compound in combination with a brominecontaining compound.
 5. An etch process as recited in claim 1, whereinthe patterned film is exposed to an iodide containing compound incombination with at least one of the group comprising fluorine,chlorine, and bromine.
 6. An etch process as recited in claim 1, whereinthe etch process deposits a passivation layer on the sidewall, thepassivation layer comprising a polymer containing silicon iodide, andwherein the passivation layer prevents the etch process fromsubstantially etching into the sidewall.
 7. An etch process as recitedin claim 1, wherein the patterned film is covered with a photoresistlayer, the photoresist layer being patterned such that sidewalls aredefined in the photoresist layer, and wherein the etch process does notsubstantially etch into the sidewalls of the photoresist layer.
 8. Anetch process as recited in claim 1, further comprising the step ofconducting a primary etch prior to exposing the patterned film toionized an iodide containing compound, the primary etch comprisingexposing the patterned film to an ionized etchant having a high etchrate, and wherein the step of exposing the patterned film to an ionizediodide containing compound further comprises a secondary etch ofexposing the patterned film to a compound containing an iodidecontaining compound, the compound containing an iodide containingcompound providing a substantially slower etch rate than the high etchrate of the ionized etchant of the primary etch.
 9. An etch process asrecited in claim 8, wherein the primary etch is conducted with anetchant comprising a chlorine-containing compound.
 10. An etch asrecited in claim 8, wherein the compound of the secondary etch comprisesSF₆ and an iodide containng compound.
 11. An etch process as recited inclaim 1, further comprising a pad oxide underlying the patterned film tobe etched and wherein the etch step etches up to and not into the padoxide so as to selectively stop on the pad oxide.
 12. An etch processwhich forms substantially anisotropic sidewalls and which has a highselectivity to oxide and photoresist, the etch process comprising:(a)preparing a patterned film to be etched on a silicon substrate of anin-process integrated semiconductor circuit; (b) conducting a primaryetch by exposing the patterned film to an ionized etchant having a highetch rate; and (c) conducting a secondary etch by exposing the patternedfilm to a compound containing ionized hydrogen monoiodide (HI), thesecondary etch having a substantially slower etch rate than the primaryetch, and the patterned film being selected from the group consisting ofsingle crystal silicon, polysilicon, silicon nitride, and refractorymetal silicide, whereby a geometric feature is formed in the patternedfilm as a result of the primary and secondary etch steps, the geometricfeature defining a sidewall.
 13. An etch process as recited in claim 12,wherein the secondary etch deposits a passivation layer on the sidewall,the passivation layer comprising a polymer containing silicon iodide,containing polymer, and wherein the passivation layer prevents etchinginto the sidewall.
 14. An etch process as recited in claim 13, whereinthe patterned film is covered with a photoresist layer, the photoresistlayer being patterned such that sidewalls are defined in the photoresistlayer, and wherein the etch process does not substantially etch into thesidewalls of the photoresist layer.
 15. An etch process as recited inclaim 14, wherein the ionized etchant used in the primary etch stepcomprises Cl₂ and NF₃.
 16. An etch process as recited in claim 15,wherein the compound of the secondary etch comprises ionized NF₃ and aniodide containng compound.
 17. An etch process as recited in claim 15,wherein the patterned film overlies a pad oxide, and wherein the etch isselective to the pad oxide such that the secondary etch does not etchinto the pad oxide so as to be selective to the pad oxide.
 18. An etchprocess as recited in claim 17, wherein the patterned film being etchedby the primary and secondary etch steps comprises polysilicon.
 19. Anetch process as recited in claim 18, wherein the primary and secondaryetch steps are conducted by ionizing the etchants thereof in a reactiveion etcher.
 20. An etch process which forms substantially anisotropicsidewalls of an in-process semiconductor circuit, the etch processhaving highly selectivity to oxide and to photoresist, the etch processcomprising the steps of:(a) preparing a patterned polysilicon film to beetched on an in-process integrated semiconductor circuit; (b) conductinga primary etch in a reactive ion etcher by exposing the patternedpolysilicon film to an ionized etchant comprising Cl₂ and NF₃ ; and (c)conducting a secondary etch in the reactive ion etcher by exposing thepatterned polysilicon film to an ionized etchant comprising NF₃ and aniodide containng compound, the secondary etch having a substantiallyslower etch rate than the primary etch, with the patterned polysiliconfilm being disposed over a pad oxide, and the secondary etch beingselective to the pad oxide such that the etch stops on and does not etchinto the pad oxide, and wherein a geometric feature is formed by theetch process, the geometric feature defining a sidewall, the etchprocess depositing a passivation layer on the sidewall, the passivationlayer comprising silicon iodide, and the passivation layer preventingthe secondary etch from substantially etching into the sidewall, andwherein the patterned polysilicon film is covered with a photoresistlayer, the photoresist layer being patterned such that sidewalls aredefined in the photoresist layer, and wherein the etch process does notsubstantially etch into the sidewalls of the photoresist layer.